In the last decade, EDM machines have been increasingly adapted to operate with numerical control (NC) systems for controlling the machining feed required to achieve a desired machining geometry. This increased use of NC systems is reflected in present-day wide-spread use in industry of highly automated traveling-wire (TW) and other "generic electrode" EDM machines capable of machining a die, mold or like highly intricate or "difficult to machine" articles more efficiently than ever before. While an NC system may also be employed for sinking-type EDM machines utilizing a formed tool electrode, the EDM capabilities have been largely expanded in the form of "generic electrode" EDM machines equipped with an NC unit. In these machines, the tool electrode is a wire, tape, cylindrical rod or like simple electrode and may be used to impart a complex shape to a workpiece with due precision.
Dies, molds or like articles have their own severe specifications to meet particular purposes so that each must be prepared from a relatively expensive blank and yet machined individually with an extremely high degree of machining precision. Furthermore, an extensive machining time ranging from an hour to days is needed to finish generating a required shape in a blank. Since a die, mold or like article which can be efficiently machined with a numerically controlled (NC) EDM machine is thus of high value and cost, any machining error during the course of machining is a critical problem.
In an NC-EDM machine, the assigned function of the NC system is to effect the machining feed required to follow up discharge-erosion at the gap and to enable the discharge-erosion to continue precisely along a programmed path. The machining feed is effected by means of a succession of drive pulses based upon digital commands programmed on a record medium so that the desired relative advancement may be performed as accurately as possible in accordance with the programmed commands. Motor means such as a DC motor or stepping motor for energization with the drive pulses is drivingly coupled with a drive member such as a leadscrew which is in turn connected to support means for the movable tool electrode or workpiece. Each individual drive pulse is, for the sake of precision, typically designed to effect an increment of the relative advancement as small as 1 .mu.m, and is furnished over a very small fraction of a second. Such successive drive pulses need to be consecutively furnished to the motor means throughout the machining operation over an extensive time period as mentioned above to maintain the machining feed precisely along the programmed feed path.
Such drive systems, however, commonly entail conversion of electrical to mechanical signals and further mechanical conversion of rotary to longitudinal signals or displacements. Thus, it has been recognized that a mechanical error may develop in the stages of conversion due, for example, to an error in the lead or pitch of a leadscrew and in backlash in various components. Furthermore, of even greater importance is the discovery that changes in gap conditions and machining parameters including electrode geometry may prevent each NC command from being precisely reflected in an actual distance of erosive material removal and hence on the corresponding machining feed or relative displacement. As a result, it is possible that minor deviations occurring from time to time for one or more of these reasons can accumulate to result in a serious machining error in the workpiece and consequent irreparable damage thereof.